In an electric power steering system which is generally equipped as a steering system of a motor vehicle, steering torque applied to a steering shaft from a steering wheel by a turning operation of the steering wheel by the driver is detected by a steering torque detecting portion. A magnestostrictive torque sensor is used as the steering torque detecting portion. The steering shaft is a rotating shaft which rotates by receiving rotating force generated by the driver who turns the steering wheel to steer the vehicle and functions in the steering torque detecting portion as its rotating shaft. In the electric steering system, a steering force assist motor is controlled to be driven in accordance with a torque signal detected by the steering torque detecting portion so as to mitigate a steering effort by the driver, giving a comfortable steering feeling to the driver.
A basic configuration of the magnestostrictive torque sensor which constitutes the torque detecting portion is shown in FIG. 11. Magnetostrictive films 102A, 102B are formed on a surface of a steering shaft (a rotating shaft) 101 of a vehicle. The magnetostrictive films 102A, 102B are provided to extend along a full circumference of the rotating shaft 101 in a circumferential direction at two locations along an axis of the rotating shaft 101 and have magnetic anisotropies 103, 104 which are opposite to each other. In the magnetostrictive film 102A, permeability changes to increase relative to clockwise torque. In the magnetostrictive film 102B, permeability changes to increase relative to counterclockwise torque. When input torque in a clockwise direction or a counterclockwise direction, which are shown by arrows 105, is applied to the steering shaft 101, the magnestostrictive torque sensor 100 detects changes in magnetic characteristics of the magnetostrictive films 102A, 102B which match torsion generated in the steering shaft 101 by detection coils 106A, 106B, respectively, in a non-contact fashion. The detection coil 106A is disposed so as to surround the magnetostrictive film 102A, and the detection coil 106B is disposes so as to surround the magnetostrictive film 102B.
FIG. 12 shows a detection principle of input torque based on the sensor configuration of the magnestostrictive torque sensor 100. A characteristic VT1 is an input torque output characteristic which is produced based on an output signal from the detection coil 106A, and a characteristic VT2 is an input torque output characteristic which is produced based on an output signal from the detection coil 106B. Since the magnetostrictive films 102A and 103A have the magnetic anisotropies 103, 104 which are directed in the opposite directions, inclinations of the characteristic VT1 and the characteristic VT2 are opposite to each other. A characteristic VT3 is an input torque output characteristic which is produced by taking a difference (VT1−VT2) between the characteristic VT1 and the characteristic VT2. Input torque applied to the steering shaft is obtained based on the characteristic VT3. Actually, a point B on the characteristic VT3 is set as an origin (an output value being 0), and an area lying on a right-hand side of the origin is referred to as a positive area, while an area lying on a left-hand side of the origin as a negative area. Information on rotating direction and magnitude of input torque applied to the steering shaft is obtained based on the characteristic VT3.
In a manufacturing method of the magnestostrictive torque sensor 100, magnetostrictive films 102A, 102B (broadly speaking, magnetostriction area portions) are formed so as to extend with an appropriate axial width along a full circumference of a surface of a rotatable rod-shaped (cylindrical) steering shaft 101 in a circumferential direction at two locations in an axial direction of the steering shaft 101, and magnetic anisotropies are provided to these magnetostrictive films 102A, 102B. A conventional method for providing magnetic anisotropies to magnetostrictive films adopts a method in which a magnetostrictive material plated portion (a magnetostrictive film) is formed through, for example, an electrolytic plating treatment so that torsional torque is applied to a shaft member (a rotating shaft). Then, stress is applied to a circumferential surface of the shaft member, and the shaft member is heated in a constant temperature bath in the stress applied condition (Patent Document 1).
Patent Document 1 proposes as a method for providing magnetic anisotropy a method in which a magnetostrictive film is plated to the surface of the steering shaft in the circumferential direction to a thickness of 40 μm, a torsional torque of 2 kgm is applied to the magnetostrictive film so as to apply stress thereto, and the steering shaft is subjected to a heating treatment at temperatures 150 to 550° C. for 10 minutes to approximately 20 hours.
In the conventional magnestostrictive torque sensor 100 shown in FIG. 11, there is a problem that failures of the magnetostriction films 102A, 102B cannot be detected accurately. The reason is that even in case a change is generated in a sensor output signal in relation to steering torque, it has not been able to determined whether the change results from a change in environment, the steering torque applied or failures of the magnetostrictive films 102A, 102B themselves.
In the configuration of the conventional magnestostrictive torque sensor 100 shown in FIG. 11, in order to enable the detection of failures of the magnetostrictive films 102A, 102B, as is shown in FIG. 13, two coils 106A-1, 106A-2, 106B-1, 106B-2 are provided for each of the two magnetostrictive films 102A, 102B (Patent Document 2). In FIG. 13, upper coils 106A-1, 106B-1 and lower coils 106A-2, 106B-2 are provided. As a result of this, by combining voltage signals which are outputted individually from the four detection coils based on predetermined relationships, a detection signal relating to steering torque and a failure detection signal can be obtained. According to the configuration, in case a failure occurs in either of the magnetostrictive films 102A, 102B, a failure of the magnetostrictive film can be detected by a failure detection signal.
Another magnestostrictive torque sensor having a failure detection construction is shown in FIG. 14 (Patent Document 3). In this magnestostrictive torque sensor 200, three magnetostrictive films 201A, 201B, 201C are formed in such a state that they are separated from each other in an axial direction of a rotating shaft 101. Two magnetostrictive films 201A, 201B which are positioned on an upper side and a lower side in FIG. 14 are magnetostrictive films to which different magnetic anisotropies are given. A failure detecting magnetostrictive film 201C is formed between the two magnetostrictive films 201A, 201B. Detection coils 202A, 202B, 202C are provided, respectively, on circumferences of the three magnetostrictive films 201A, 201B, 201C. A signal regarding steering torque is taken out based on two detection signals which are outputted from the two detection coils 202A, 202B. Further, a failure detection signal is taken out based on three detection signals which are outputted from the three detection coils 202A, 202B, 202C.
In the conventional magnestostrictive torque sensors, in the case of the configuration in which the steering torque detection and the magnetostrictive film failure detection can be executed at the same time, with the magnetostrictive films provided at the two locations, a total of four detection coils is required. The configuration has a problem that the number of components is increased, which increases, in turn, the manufacturing cost and manufacturing processes. In addition, in the event of the magnetostrictive films being formed at the three locations, a total number of detection coils becomes three. However, in an actual manufacturing process, the composition of iron constituent (Fe) of the magnetostrictive film which is positioned in the middle has to be lower than those of the other upper and lower magnetostrictive films, leading to a problem that the number of manufacturing processes is increased. Further, an axial dimension of the area on the steering shaft (the rotating shaft) where the magnetostrictive films are formed becomes long, leading to a problem that the magnetostrictive film area is enlarged.    Patent Document 1: JP-A-2002-82000    Patent Document 2: JP-A-2006-64445    Patent Document 3: JP-A-2007-101422